Imaging apparatuses for converting optical image to electric signals, such as digital still cameras, and image-inputting apparatuses such as facsimiles, scanners, etc. have recently got widely used. As electronic imaging apparatuses comprising imaging devices such as charge-coupled devices (CCD), etc. have been finding wider applications, problems have become serious by foreign matter such as dust, etc. existing in an optical path to a light-receiving surface of the imaging device, which tends to appear in the resultant image.
For instance, in the case of a digital single-lens reflex camera having an exchangeable shooting lens, dust, etc. tend to intrude into a mirror box when the shooting lens is detached. Also, because a mechanism for controlling the movement of a mirror and the closing of a diaphragm shutter is operated in the mirror box, dust is likely to be generated in the mirror box. In the case of the image-inputting apparatuses such as a facsimile, a scanner, etc., foreign matter such as dust, etc. is likely to be generated when a manuscript is fed or moves through a manuscript-reading unit, resulting in attaching to a light-receiving surface of the CCD or to a manuscript-placing platen glass, etc. Foreign matter attached to the imaging device surface, etc. is usually blown off by an air blower, etc. However, the blown foreign matter remains in the imaging apparatus.
The digital still camera is provided with an optical filter for controlling spatial frequency characteristics near an imaging device, and a birefringent quartz plate is generally used as the optical filter. However, because quartz has a piezoelectric effect, it is easily charged by vibration, etc., and the resultant charge is less likely to disappear. Accordingly, when foreign matter floats in the camera by vibration, airflow, etc. caused by the operation of the camera, it tends to be attached to the charged optical filter. The attached foreign matter should be removed by blowing air to the optical filter, or wiping or vibrating the optical filter. However, the mechanical removal of dust is expensive, makes the apparatus heavier, and consumes more electric power.
JP 2006-163275 A discloses an optical part constituted by an optical lowpass filter of quartz and an infrared-cutting filter, and having a coating for preventing the attachment of foreign matter on the surface. When this optical part is disposed in an optical path in an imaging apparatus, the light passes through the foreign-matter-resistant coating, the infrared-cutting filter the optical lowpass filter, and enters into the imaging device. Because the optical part surface is provided with the foreign-matter-resistant coating on the surface, it is presumed that the foreign matter is less likely to appear in the resultant image. However, there is still some foreign matter appearing in the image. As a result of intensive research, it has been found that this is mainly caused by scratches on the optical lowpass filter. As demand is mounting for clear image, an imaging apparatus free from adverse influence on the image is increasingly desired.